Western Spruce Budworm, Choristoneura occidentalis

The western spruce budworm is an important native defoliator of interior Douglas-fir. Outbreaks have been recorded in BC since the early 1900's to present in interior BC forests. Periodically, population levels reach outbreak proportions. In 1987, over 800,000 ha were under attack by the budworm, mostly in the southern interior of the province. Budworm outbreaks may be sustained for 25 years or more.

Host trees: Primarily Douglas-fir, with other tree species such as the true firs, larch and to a lesser degree, spruce, also impacted by the western spruce budworm.

Description and life cycle: The western spruce budworm requires one year to complete its life cycle.

Egg: Budworm moths mate and lay eggs in late July through August. The eggs are laid in masses (up to 150) on the underside of needles, overlapping like fish scales. They are bright green when laid, measuring slightly less than 1 cm in total length, and become translucent when empty. They hatch within 12 days of being laid.

Larva: The western spruce budworm goes through 6 larval instars during its life cycle. Early instar larvae do not feed, but overwinter as 2nd instars in silken webs (hibernaculae) on tree branches under bark scales, and in flower and needle scars.

As buds swell in the spring (late May), larvae migrate and begin to feed, initially mining the needles, and then moving onto the developing buds and cones. With bud flush, larvae feed on the new shoots, subsequently moving onto older foliage, spinning loose webs around new foliage and feeding within them during the fifth and sixth instars. Later instar larvae range from 16-30 mm long. The upper body is yellow-brown with conspicuous paired ivory coloured spots on its body segments. The head capsule is brown. Feeding continues through late June to early July, at which time the larva pupates.

Pupa: The larva pupates from late June to mid-July within the webbed foliage. The pupa is 12-16 mm long, being broad at the head end, tapering to a point at the rear. It is reddish brown in colour. The pupal stage lasts on average 2 weeks.

Adult: Adults emerge between mid-July and early August, mate and lay eggs on host foliage. They die within two weeks of emergence. They are inconspicuous looking insects, mottled grey to rusty brown in colour, with a wingspan of 24-30 mm.

The greatest impact from budworm is among suppressed and intermediate trees. Repeated budworm defoliation causes tree mortality over large areas, reduction of growth rates and reduced lumber quality. Sustained attack results in complete defoliation in 4 to 5 years. Once an infestation has subsided, defoliated trees take several years to regain a full foliage complement, and therefore radial growth rates require several years to attain normal growth following defoliation by the budworm. Successive years of defoliation in stands may predispose trees to other insects and pathogens. Tree mortality may continue due to root disease, bark beetles, loss of vigour and other causes.